FIG. 12 is a diagram illustrating a configuration of a conventional reflective liquid crystal display device. In the figure, a liquid crystal display panel 1001 consists of a pixel matrix 1005, a scan driver circuit 1008, and a data driver circuit 1009, while each pixel constituting the pixel matrix 1005 is configured by a pixel TFT (thin film transistor) 1002, a liquid crystal capacitor 1003, and an additional capacitor 1004. The gate of each TFT 1002 in the pixel matrix 1005 is connected to a respective scan line 1006, and scanned by the scan driver circuit 1008. The source of each TFT 1002 in the pixel matrix 1005 is connected to a respective data line 1007, through which signals from the data driver circuit 1009 are written into each liquid crystal capacitor 1003. Provided as external circuits are: an image signal processing circuit 1016 for processing externally inputted image signals; a driver control circuit 1017 for controlling the scan driver circuit 1008 and the data driver circuit 1009; and an LED driver circuit 1021 for driving according to synchronization signals LEDs 1020 used as a light source for a front light 1019 installed on the front display side of the liquid crystal display panel.
Next, operations will be explained. The inputted image signals are inputted into the image signal processing circuit 1016, and then outputted to the data driver circuit 1009 as RGB data with a predetermined timing. In addition, the driver control circuit 1017 generates, according to the inputted synchronization signals, control signals for controlling the scan driver circuit 1008 and the data driver circuit 1009.
In the data driver circuit 1009, to begin with, shift pulses are generated by a start pulse STH and a shift clock signal CLKH that are inputted. Then the data for a single line is developed by the shift pulses sequentially latching RGB data for the single line. After being latched further with a common latch pulse LP, the latched RGB data is converted into analog signals and transferred to each data line.
Meanwhile, in the scan driver circuit 1008 shift pulses are sequentially generated by a start pulse STV and a shift clock signal CLKV that are inputted, and serve as signals for scanning the scan lines 1006.
In each pixel circuit that constitutes the pixel matrix 1005, when the scan line 1006 connected to the TFT 2 is scanned by the scan driver circuit 1008, the TFT 1005 turns conductive. Then the analog signal for the display line outputted to each data line 1007 from the data driver circuit 1009 is applied through the TFT 1002 to the liquid crystal capacitor 1003 and the additional capacitor 1004. The drain of the TFT 1002 is connected to a reflective electrode (not shown in the figure). A voltage in accordance with the difference between the voltage being applied via the TFT 1002, and the voltage of a transparent opposing electrode is applied to the liquid crystal capacitor 1003, and by the liquid crystal optically responding to the voltage, the reflectance of the RGB dots for each pixel varies in accordance with the RGB data. Thus, displaying onto the display surface is carried out.
Meanwhile, the LEDs 1020 that are the light source for the front light 1019 installed on the front side of the liquid crystal display panel 1 are driven by the LED driver circuit 1021 so that a forward current can flow, and then emit light. The front light 1019 is generally used as an auxiliary light source under a use environment in which, particularly in a room, for example, light incident from outside into the liquid crystal display panel 1001 is feeble.
Moreover, for example in another conventional reflective liquid crystal display device shown in Japanese Patent Laid-Open No. Hei 5-158034, between a pair of polarizers a liquid crystal display element is disposed to configure the device. In addition, between the liquid crystal display element and the polarizer an optical guide plate is disposed with an airspace intervening, and lamps are set up respectively along the outer sides of the opposing edge faces of the optical guide plate.
Thus it is disclosed that a light source device including an optical guide plate and a light source can be disposed on the front (display surface) side, whereby when the light source is on, even and favorable lighting is achieved, and when the light source is off, the optical guide plate turns transparent not to interfere with the incident light from outside, and to achieve favorable display.
Furthermore, in a conventional transmissive liquid crystal display device shown in Japanese Patent Laid-Open No. Hei 4-174819, it is disclosed that a light detection signal output from a separately provided photodetector is inputted, and that the device includes memory for beforehand storing a correlation between the light detection signal and the light intensity, and a controller 5 for adjusting the light intensity of a backlight in accordance with the signal output from the memory.
There has been a problem that in the conventional reflective liquid crystal display device including a front light (a light source unit installed on the front (display surface) side of the liquid crystal display device and including an optical guide plate and a light source) as mentioned above, because the luminous intensity of the front light is constant, if the external light illuminating from outside of the device varies, it causes the display surface illuminance to vary, whereby the display luminance varies.
There has been another problem that under a relatively bright use environment, even if an adequate display luminance is achieved with external light only, because the amount of emission from the front light is constant, the front light consumes electric power in vain.
There has been a further problem that the transmissive liquid crystal display device with a backlight requires memory for storing the relation between the result of detecting external light intensity and the amount of backlight emission, whereby the scale of a circuit increases.
The present invention has been made to resolve the conventional problems above-mentioned, and aims to provide a reflective liquid crystal display device in which even if the light intensity of the external light illuminating from outside of the device varies, the amount of front light emission can be automatically controlled to maintain automatically an appropriate display luminance.